Hitboxes are areas that players cannot travel through.
Do you have a wall and want the player to not be able to go through it? Use a hitbox. Is there a tree and you want the player to stop when they run into the trunk? Use a hitbox!
When coming up with the design for your hitbox, you want to think about where the area of the element is on the ground.
For example, on this tree, the trunk is sitting on the ground, so a good hitbox would only block the tree trunk.
For a tree, try placing the hitbox just around the base of the trunk rather than around the entire tree.
If you were to create a hitbox around the entire tree (the "bad" screenshot above), this would mean the player could not go behind the tree.
In general, smaller hitboxes feel better, since it gives the player more room to move around and go behind the element.
Experiment with your hitboxes by running around them until they feel right!
Hitboxes blocking the bottom of tree trunks, a launch pad and a waterway.
Hitboxes blocking the bases of several objects in an environment, including plants and buildings.
Hitboxes blocking portions of a collection of trees, lamps and a large building.